The present invention relates, in general, to electronic packages and, more particularly, to electronic packages including a heat sink for heat dissipation from a semiconductor die.
Power transistors typically generate considerable heat which must be transferred from a semiconductor die to the external environment. Usually, this is accomplished by placing the die on a heat sink which is used as a medium to transfer heat from the die to, for example, an underlying printed circuit board. As is conventional, a plastic package typically encapsulates the die and a portion of the heat sink to provide what is known as a power electronic package.
In the formation of a power electronic package, a leadframe is used to support the package leads while the semiconductor die and a portion of the heat sink are encapsulated by a molding compound, which is cured to provide the final package. Typically, a single leadframe is used to mold many such packages simultaneously. In a prior approach, a continuous strip of heat sinks is supported below the leadframe so that a row of final packages may be molded. The heat sink strip is supported beneath the leadframe during the molding process by attaching portions of the strip to mounting tabs on the leadframe by a technique known as mechanical staking. This staking requires that regularly-spaced portions of the heat sink strip be elevated to provide an offset between the strip and the leadframe. However, staking requires considerable leadframe area that could otherwise be used for forming additional packages and also requires that the thickness of the heat sink strip be sufficient to withstand the deformation applied to the strip to elevate the regularly-spaced portions used for staking. One problem with a thick heat sink strip is that the final package thickness is increased. Also, a thicker heat sink strip is more difficult to cut after molding, and this cutting puts significant stress on the die, which can result in a cracked die or a loss of adhesion between the die and the heat sink.
Another problem with the use of a continuous strip of heat sinks is that a mismatch of mechanical stress, thermal expansion, or pitch between the strip and the leadframe often results in induced coil set and camber or other manufacturing problems, which thereby prevents the formation of reliable packages. Another limitation with the use of a continuous heat sink strip is the inability to form a four-sided power package with an exposed heat sink. This is so because leads from the leadframe cannot extend over those portions of the heat sink strip used to connect each of the individual heat sinks to one another. Instead, with this prior approach only one, two, or three-sided packages can be formed.
Accordingly, there is a need for a power package that uses less area for staking, uses a thinner heat sink material, reduces manufacturing problems associated with mismatches between the heat sink strip and the leadframe, and permits the formation of four-sided packages with exposed heat sinks.